1. Field of the Invention
The present invention relates to an image processor, an image processing method, and an image forming apparatus.
2. Description of the Related Art
When performing image processing, some image forming apparatuses employ a technique called tone processing. In the tone processing, multilevel input image data are converted to image data having less levels, such as a binary value. The tone processing allows reduction in the amount of data.
The tone processing includes a dither method, an error distribution method, or a combination of these two methods. The dither method is excellent in graininess, while the error distribution method is excellent in sharpness.
In the error distribution method, multilevel data is quantized by a quantization processing. The quantization processing uses a quantization threshold. As in a dither processing, a value corresponding to the pixel position is used in the quantization processing. Several techniques have already been proposed to improve deterioration in the texture due to irregular dot connection in the error distribution method. Japanese Patent Application Laid-open No. 2001-352448 discloses a conventional technique in which a quantization threshold is periodically fluctuated.
In the tone processing, stable dot formation can be achieved by suppressing occurrence of isolated dots. As a result, various techniques that can suppress the occurrence of isolated dots have been studied and disclosed by various people. Japanese Patent Application Laid-Open No. H11-041464 and 2000-022944 discloses such techniques.